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Interpretive Summary: Cereal rye cover crop mulches can suppress weeds in a reduced-tillage grain production system. However, this suppression is not consistent and often requires supplemental weed management tactics. The inconsistency in suppression can be, in part, dependent on the initial weed infestation level. This study assessed the weed suppressive ability of varying cereal rye mulch levels alone, across a range of initial weed seedbank densities, and in an integrated system of herbicide or high residue cultivation. Rye biomass alone provided ample weed suppression when weed infestation levels were low (ambient or low). As the manipulated weed infestation level increased, a greater amount of rye biomass was needed to achieve reasonable levels of weed suppression. Irrespective of management system, as weeds increased, supplemental weed control (e.g., glyphosate application or cultivation) was needed to achieve acceptable levels of weed suppression. Chemical weed control was more consistenly effective at high initial weed seedbank densities. Thus, under the condition of this experiment, at low weed infestations, a rolled rye cover crop can serve as an effective weed suppression tactic while at higher infestations it should be viewed as one component in a multi-tactic approach. Longer-term systems level studies are needed to assess the population dynamics of the summer annual weeds studied herein. It will also be important to assess the impact of such practices on perennial weeds as they are known to increase in low-herbicide and low tillage environments.

Technical Abstract: Rolled cover crop mulches can suppress weeds in subsequent cash crops, reduce the need for
herbicides and allow organic no-till cash crop establishment. This study investigated the weed
suppressiveness of a cereal rye (Secale cereale L.) cover crop mulch across varying weed seedbank density. Cereal rye was seeded at two dates in the fall and terminated at five dates in the spring to create biomass ranging from 100 to 1600 g m-2. The first three termination dates included both herbicide and rolling of the rye (conventional), while later three dates were only rolled (organic). Soybean (Glycine max (L.) Merr.) was no-till planted after rye termination, and weed biomass and soybean yield were assessed. Spring termination date more strongly affected cereal rye biomass than fall planting date; a termination delay of 5 to 15 days compensated for a planting delay of 30 days. Weed biomass generally declined with increasing cereal rye biomass, and this relationship was stronger at higher weed seedbank densities. Supplemental weed control reduced weed biomass compared to no supplemental control and post herbicide was more effective than cultivation. While increasing cereal rye biomass was associated with a decline in soybean yield in 2009, it did not consistently impact soybean stand. Instead soybean stand establishment appeared to be impacted by high cover crop biomass and changing edaphic conditions at planting. Future research should focus on improved technology for direct seeding in high residue environments and developing longer term cropping systems less reliant on tillage and herbicides.